Fuel tanks of an aircraft can be located in the wing structure as integral fuel tanks. Additional fuel tanks are sometimes located in the fuselage or tail of the aircraft to extend the flying range of the aircraft.
With general design, each wing is attached below or at a lower portion of the fuselage. This configuration provides a difference in height between a fuselage tank and each wing tank. When plural tanks are connected, fuel pressure head is applied from the uppermost fluid level of the upper tank to the bottom of the lower tank.
Aircraft structure should be designed to withstand the fuel pressure as well as other maneuver loads and aerodynamic loads. When maneuver acceleration is applied in the vertical direction, the fuel pressure increases proportionately. For example, vertical acceleration in emergency landing is specified as 6.0 g downward for normal category airplanes. The fuel tanks should withstand the generated pressure under such high-acceleration conditions. If the upper tank is hydraulically connected to the lower tank, it will require additional reinforcement to the lower fuel tank structure.
A general solution for the aforementioned issue is usually provided in one of two methods. A first method is to disconnect two tanks by a shutoff valve, and to open the shutoff valve as required when fuel in the lower tank is consumed. The second method is to transfer fuel from the upper tank to the lower tank by a transfer pump. In either method, level sensors and manual or automatic electronic control devices are required. This makes the fuel management system more complicated.
Another example of an auxiliary fuel tank system includes upper aft auxiliary fuel tanks and a forward auxiliary fuel tank. The aforementioned fuel tank system also includes a center wing tank, which is the aircraft's existing fuel tank. The sequence of gravity fuel transfer from the auxiliary tanks to the center wing tank is set in three stages. First, the aft auxiliary fuel tanks transfer fuel into the center wing tank until the aft auxiliary fuel tanks become empty. Next, the forward auxiliary tank transfers fuel into the center wing tank until the forward auxiliary tank becomes empty. Finally, the center wing tank transfers all remaining fuel into the wing tanks until the center wing tank becomes empty.